fix tiny android builds (add mksh)

Change-Id: I0cf5068b6e9b350259009c5ecf7f2946d05f7b2d
diff --git a/core/main.mk b/core/main.mk
index 571e779..1975bdb 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -467,6 +467,7 @@
 	build/libs \
 	build/target \
 	build/tools/acp \
+	external/mksh \
 	external/yaffs2 \
 	external/zlib
 else	# !BUILD_TINY_ANDROID